The present invention relates to an anchor for fastening in drill or bore holes formed in supporting structures, for example of concrete.
Anchors for anchoring in pre-drilled bore holes of the type under discussion have been known. One of such anchors is disclosed, for example in applicant's U.S. Pat. No. 3,837,257. The anchor of the foregoing type includes an expansion sleeve having an expanding slotted portion and driven onto an expander body by means of an elongated threaded stay bolt screwed into the expander body and accessible from the outside of the bore hole. A torque applied to the head of the stay bolt draws the expansion sleeve onto the conical surface of the expander body. An intermediate sleeve of plastics is arranged between the outer support of an object being secured to the supporting structure, for example of concrete, and the expansion sleeve. This plastic sleeve forms a bearing zone.
Conventional anchors are fastened in the supporting structure in such a manner that the expander cone is drawn into the expansion sleeve whereby the expansion sleeve is supported against the abutment or support of the stay bolt either immediately or via a spacer sleeve. The expander body can be connected to the bolt by means of a thread as mentioned above or it can be pressed against the bolt which has a nut as an abutment or outer support.
The insertion of such an anchor into the pulling zone of the anchor base requires high after-expansion capabilities in order to avoid decrease of holding forces due to widening of the bore hole caused by the formation of cracks. A satisfactory after-expansion can be obtained by a flat cone of the expander body. However, with such flat cone the rigidity of the anchor base is not completely used because the expander body is prematurely drawn through the expansion sleeve. This effect occurs even sooner when the bore hole is widened by a crack, as compared to the initial diameter of the bore hole.
With a very steep angle of the conical surface of the expander body there is a danger that the anchor can expand non-sufficiently because high torque required for driving of the expander cone into the expansion sleeve can not exceed a permissible load on the stay bolt. The failure of such anchoring normally results in that the entire anchor falls out from the bore hole.